1. Field of the Invention
This invention relates to the field of amplifiers, and particularly to amplifier circuits capable of providing large output swings on demand.
2. Description of the Related Art
In some applications, an amplifier must maintain an average output, and drive more-or-less symmetrically about the average value. Such amplifiers are limited in their drive capability by the asymmetry of the supply voltages, or, in the case of a single supply amp, by the common mode voltage.
One such application is that of an LCD panel in which the amplifier provides the panel's “VCOM” voltage. A desired VCOM voltage (VSET) is applied to the amp's non-inverting output; feedback from the panel applied to the inverting input causes the amplifier to sweep out the charge resulting from pixel voltage reversal. One factor which limits the amplifier's performance in this application is the settled value of the force voltage required by the panel. If the amplifier which drives VCOM is powered by a single supply, the maximum negative output voltage swing is limited to the nominal value of the VCOM voltage with respect to ground. The limited swing extends the time it takes the VCOM amp to restore panel charge through the effective series impedance at the force terminal. The problem is more evident for negative swings; since the VCOM voltage is generally less than half the amplifier supply voltage, the available positive voltage swing is greater.
FIG. 1 illustrates the issue with a block diagram. A greatly simplified LCD panel 10 is represented by the block labeled LOAD. In practice, a number of DC voltages are applied to the panel, but for purposes of this illustration they can be lumped together at the ground terminal. A bias voltage, Vout, is applied to the panel's VCOM input by amplifier 12, which is typically capable of swinging its output voltage from rail to rail. In operation, signals are applied to various panel inputs to charge pixel and other capacitances. The net effect (as seen from the amplifier) of these signals is represented by VDRIVE, a time varying drive voltage. This source is shown isolated from panel 10 by a capacitance C. Although the panel is basically capacitive, so there is little or no net current in the various connections, changes in the pixel voltages from row to row as an image is scanned in result in charge being delivered to the panel from VDRIVE. Typically, this charge alternates, row to row, and averages to zero over several lines.
However, the amount of charge delivered to one row may be substantial and an image charge must be supplied by the VCOM amplifier in order for the pixel capacitance to be charged to the correct voltage. Since the effective time constant of the distributed R-C of the panel may be longer than the time allotted to write a row of pixels, simply maintaining the voltage applied to VCOM may not suffice to charge all the pixels to their desired final voltage.
One remedy for this problem is to sense the disturbance of the LCD panel network (via the panel's SENSE node) and use feedback to drive the VCOM voltage so as to rapidly sweep out the image charge from the pixel voltage changes. For example, assume that for a given row, the effect of all the pixel voltage changes is as if VDRIVE went positive. This would drive the VCOM return path through the pixel capacitances and the distributed resistance, tending to drive the SENSE node positive. The SENSE node is coupled to the inverting input of amp 12 via a resistance R1, which is also driven positive. This causes Vout to swing negative, in opposition to the drive from VDRIVE, and rapidly provide the majority of the image charge required to set the pixel voltages. Vout is also fed back to the inverting input via a resistance R2; this feedback causes the amplifier to recover to the desired VCOM voltage after the large output voltage pulse.
The signs of the pixel voltages are typically arranged so that they will alternate from row to row, so that the next row will require VDRIVE to go negative, restoring most of the charge from the previous line, and requiring VCOM to be driven positive.
Some picture content may call for very large amounts of charge per row, and therefore large peak currents. Since the input of the panel is more-or-less resistive, a large signal swing may be required to replace the charge for these rows. For this reason, a rail-to-rail amplifier is usually preferred in order to get the greatest possible voltage swing. For reasons of system architecture, a grounded single supply amplifier is generally preferred. Here, the maximum output voltage swing the amplifier can supply is limited to the difference between the nominal settled VCOM voltage and the supply rail to which the output swings. The amplifier's positive supply—and thus the positive output swing—can be made arbitrarily large. However, the negative voltage swing is limited to ground potential. The desired VCOM voltage is constrained by the biasing needs of the panel to a relatively small voltage. For example, if VSET is set to 4V and the positive supply voltage is 12V, amplifier 12 can drive panel 10 with a positive pulse approaching 8V in amplitude, while the negative pulse is limited to 4V.
For some difficult display content, the charge required may be greater than the ˜4V signal that amplifier 12 can provide in the time allotted, resulting in the pixels being left with the wrong voltage stored on them. This creates a problem which is particularly noticeable when the picture has areas of uniform color which overload the VCOM amp, and other areas of less troublesome picture content which do not overload the amplifier. The “good” picture content reduces the charge requirements in the rows where it appears, so that the background is rendered properly in those rows, while in rows consisting entirely of the troublesome pixels, pixel voltages do not settle completely to the correct values, and these rows appear different from the rows with additional content. This undesirable phenomenon is known as “banding” and is quite noticeable.